It's very simply. Put the radishes in a jar with a bit of salt. I also like to ad one small piece of ginger. Cover it with water and use some sort of weight to make sure they are not exposed to air - I use a stone the size of the mouth of the jar. Wrap it in a towel or put in a dark place. It will be ready in 3-7 days, depending on your personal tastes. When done correctly the water will have a slightly bubbly effect. The taste will be sour, not spoiled.